NEW YORK (Reuters Health) - The number of men and women living with chronic bronchitis and emphysema - collectively known as chronic obstructive pulmonary disease (COPD) - is on the rise, Canadian study findings hint.

COPD - often tied to smoking - develops when tiny air sacs lining the lungs lose elasticity and become increasingly unable to exchange air. Although manageable with medications, COPD is not curable. Symptoms of impaired breathing and mucous-laden coughing worsen over time.

In the U.S., COPD is the fourth leading cause of death. According to the National Heart Lung and Blood Institute, an estimated 12 million Americans have been diagnosed with the disease, and another 12 million likely have it but have not been diagnosed.

In Ontario, Gershon and colleagues found that most COPD cases are among men, but that women appear increasingly saddled with the disease, the investigators report in Archives of Internal Medicine.

In 1996, 7.8 percent of Ontario residents 35 years and older had COPD. By 2007, 9.5 percent did.

Improved treatments have helped people live longer with COPD, which means more cases at any given time, according to the researchers. This, and declines in smoking rates over time, appear to have led to declines in COPD mortality: Deaths due to COPD dropped from about 6 percent in 1996 to about 4 percent in 2007.

In women, the increase in COPD from 1996 to 2007 was about twice what it was in men -- about 33 percent versus 13 percent.

By 2007, more than 360,000 women and nearly 349,000 men had COPD, but Gershon noted the reason for this increase in women remains unclear.

Despite some fluctuations, however, there are still a lot of people suffering from COPD. In 2007, Gershon's team identified nearly 56,000 new COPD cases.